Recently, a surface emitting laser for emitting laser light perpendicularly to a substrate has been studied extensively because the surface emitting laser can be advantageously arrayed two-dimensionally, compared with an edge emitting laser.
Further, recently, a surface emitting laser using a photonic crystal has been studied.
As such a surface emitting laser using a photonic crystal, Japanese Patent Application Laid-Open No. 2000-332351 describes a configuration in which an active layer is sandwiched between cladding layers having a refractive index lower than that of the active layer and a photonic crystal having a refractive index profile is formed in an in-plane direction in the cladding layers.
In the surface emitting laser, light guided in the in-plane direction in the active layer resonates in a plane due to the second-order diffraction function of the photonic crystal, and laser light is taken out in a perpendicular direction by the first-order diffraction of the photonic crystal.
Further, Japanese Patent Application Laid-Open No. 2003-273456 discloses a two-dimensional photonic crystal surface emitting laser in which a reflector and a reflector structure with a period of about a half are provided in the periphery horizontal to the crystal plane of the photonic crystal to suppress the loss of light in the horizontal direction, whereby the use efficiency of light is enhanced.
When the surface emitting laser using a photonic crystal, which is disclosed in Japanese Patent Application Laid-Open No. 2000-332351, is arrayed, in particular, in the case of high-density arraying (for example, an array pitch is 100 μm or less) to be desired industrially, the following problem arises.
More specifically, in the case where surface emitting lasers adjacent to each other, which form the surface emitting laser array, are connected through the active layer, the waveguide mode in one surface emitting laser is guided to the other surface emitting laser through the active layer. As a result, for example, the characteristics of the other surface emitting laser are degraded by such light guiding.
Hereinafter, the above-mentioned problem is described more specifically with reference to the drawings.
FIG. 2 is a schematic view of a surface emitting laser array 200 obtained by arraying a surface emitting laser using a photonic crystal, which is disclosed in Japanese Patent Application Laid-Open No. 2000-332351, which is viewed from a direction perpendicular to a substrate.
The laser light from a surface emitting laser 201 that resonates in a plane by a photonic crystal 210 is partially guided in the active layer to leak out of a region of the photonic crystal. Consequently, leaking light 220 reaches the active layer of another surface emitting laser 202 to influence the characteristics of another surface emitting laser.
More specifically, in the case where the laser oscillation wavelengths are the same between those surface emitting lasers, resonators may interfere with each other.
Further, even in the case where the oscillation wavelengths are different from each other, if the leaking light is absorbed by the active layer of another surface emitting laser, the leaking light influences the carrier distribution and consequently influences the output characteristics of another surface emitting laser (optical crosstalk).
The cause of those problems is further described with reference to FIG. 3.
FIG. 3 is a schematic view illustrating a cross-section of the surface emitting laser array 200 illustrated in FIG. 2, which is taken along a line 3-3′ of FIG. 2.
The refractive index of an upper clad (slab layer 320 and cladding layer 330) and a lower cladding layer 340 sandwiching the active layer is lower than that of the active layer.
Therefore, there is a resonance mode that is guided in the active layer due to a total refection in a photonic crystal region 210 (i.e., region of the surface emitting laser 201).
Herein, an active layer 310 illustrated in FIG. 3 includes an active medium such as a quantum well and a spacer layer that adjusts the position of the active medium and the light intensity distribution of the resonance mode.
In the slab layer 320 of the upper clad, the photonic crystal 210 is formed.
The cladding layer 330 of the upper clad may be air.
Herein, even in a region 205 outside the surface emitting laser, the active layer 310 is sandwiched between clads with a low refractive index, and hence there is a waveguide mode at the resonance wavelength of the surface emitting laser 201.
When light intensity distributions of the respective waveguide modes overlap each other inside and outside the region of the surface emitting laser, the laser light resonating in the surface emitting laser 201 is guided to the region 205 outside the surface emitting laser, and further reaches the active layer in another surface emitting laser 202.
As a result, the laser light may influence the characteristics of another surface emitting laser as described above.
Thus, it is necessary to break the connection between the surface emitting lasers adjacent to each other, which form the surface emitting laser array, through the active layer, thereby suppressing light in a waveguide mode of one surface emitting laser from being guided to another surface emitting laser.
In the case of disconnecting the active layer between the surface emitting lasers adjacent to each other for this reason, such a disconnection becomes an obstacle for integrated high-density arraying, and a crystal defect may occur from the disconnected surfaces. This may also lead to the increase in the number of process steps.
Further, when the active layer is disconnected, if the disconnected width between the surface emitting lasers adjacent to each other is small, it is difficult to stop the guiding of light to another surface emitting laser.
In such a case, it is considered to apply the configuration of Japanese Patent Application Laid-Open No. 2003-273456 between the surface emitting lasers adjacent to each other. This is because, it is considered that, the suppression of a loss of light in the horizontal direction of a laser element also leads to the suppression of guided light to another element simultaneously.
However, the application of the configuration of Japanese Patent Application Laid-Open No. 2003-273456 leads to the following problem.
That is, for arraying laser elements, a reflector structure is provided between the surface emitting lasers adjacent to each other, but the provision of such a reflector structure hinders high-density arraying.
The above-mentioned problem is described specifically with reference to the drawings.
FIG. 4 is a schematic view in which a reflector structure 450 is provided at a position for surrounding the surface emitting lasers 201 and 202.
Further, FIG. 5 is a schematic view in which a photonic crystal structure (560 of FIG. 5) having a reflection function (having a photonic band gap) in a horizontal direction is provided as a reflector.
In the case where the reflector structure 450 is provided in the surface emitting laser array 400 arrayed as illustrated in FIG. 4, the provision of the reflector structure 450 hinders high-density arraying. In particular, when the reflector structure 450 is provided, it is difficult to suppress the transmission of light from a laser element to another laser element without disconnecting the active layer that is a waveguide.
Further, the disconnection of the active layer causes the above-mentioned problems.
Further, as illustrated in FIG. 5, in the case where a photonic crystal structure 560 is provided as a reflector in a surface emitting laser array 500 that is arrayed, it is necessary that the photonic crystal structure 560 have a frequency to some degree, in order to allow the photonic crystal structure 560 to function sufficiently as the reflector.
For this reason, it is necessary to set a wider place for the reflector, which is considered to hinder higher-density arraying.
In the case where the cladding layer 330 is air, wiring for electrically driving each surface emitting laser can be placed in the vicinity of the photonic crystal 210.
However, if the reflector structure as described above is provided, compared with the case where there is no reflector structure between elements and the surface of the slab layer 320 is flat, it is more difficult to install electric wiring necessary in the case of electrically driving each laser element of the surface emitting laser array. Thus, also in this respect, high-density arraying may be become difficult.